What are Kuiper Belt Objects?




Our Solar system is surrounded by two large formations of objects.
• The Kuiper Belt
• The Oort Cloud
The Oort Cloud is discussed in the subquestion "What are the origin and fate of the Oort Cloud?" by Chris Boersma and Niels Bos. (Oort Cloud)
Therefore we will only discuss the Kuiper Belt.

What is the Kuiper Belt?
Since 1992, scientists have noticed a large population of small bodies orbiting the sun beyond Pluto. There are at least 70,000 "trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Pluto (at 39 AU) to 50 AU. There may be many more similar bodies beyond 50 AU, but these are presently beyond the limits of detection. Observations show that the trans-Neptunians are mostly confined within a few degrees of the ecliptic, leading to the realization that they occupy a ring or belt surrounding the sun. This ring is generally called the Kuiper Belt.
The Kuiper Belt is important for the study of the planetary system on at least two levels.
First, it is likely that the Kuiper Belt objects are extremely primitive debris from the formation of the Solar system. The inner, dense parts of the proto-planetary disc condensed into the major planets, probably within a few millions to tens of millions of years. The outer parts were less dense, and accretion progressed slowly. Evidently, a lot of small objects were formed. So the composition of KBO's can tell us a lot about the conditions in the proto-planetarydisc
Second, it is widely believed as the Kuiper Belt is the source of the short-period comets. It acts as a reservoir for these bodies in the same way that the Oort Cloud acts as a reservoir for the long-period comets. The study of the trans-Neptunians is a rapidly evolving field, with major observational and theoretical advances in the last few years.


Moving Kuiper Belt Object
Obtained from: http://www.ifa.hawaii.edu/faculty/jewitt/kb.html


What are the properties of Kuiper Belt Objects?
There are three types of KBO's: Plutino's, Classical and Scattered KBO's.

Plutino's
A surprising result of the new observational work is that many of the distant objects are in or near the 3:2 resonance with Neptune. This means that they complete 2 orbits around the sun in the time it takes Neptune to complete 3 orbits.The same resonance is also occupied by Pluto.
To mark the similarity with Pluto, these objects are called "Plutinos" (little Plutos). Probably, the 3:2 resonance acts to stabilize the Plutinos against gravitational disturbances by Neptune. Resonant objects in elliptical orbits can approach the orbit of Neptune without ever coming close to the planet itself, because their perihelia (smallest distance from the sun) avoid Neptune. In fact, it is well known that Pluto's orbit crosses inside that of Neptune, but close encounters are always avoided. This property is also shared by a number of the known Plutinos (e.g. 1993 SB, 1994 TB, 1995 QY9), further enhancing the dynamical similarity with Pluto. Approximately 35% of the known trans-Neptunian objects are Plutinos.
A few more are suspected residents of other resonances (e.g. 1995 DA2 is probably in the 4:3).
By extrapolating from the limited area of the sky so far examined, the estimated number of Plutinos larger than 100 km diameter is of order 25,000.
Pluto is distinguished from the Plutinos by its size: it is the largest object identified so far in the 3:2 resonance.
How did the 3:2 resonance come to be so full?
An exciting idea has been explored by Renu Malhotra. Building on earlier work by Julio Fernandez, she supposes that, as a result of angular momentum exchange with planetesimals in the accretional stage of the solar system, the planets underwent radial migration with respect to the sun. Uranus and Neptune, in particular, ejected a great many comets towards the Oort Cloud, and as a result the sizes of their orbits changed. As Neptune moved outwards, its mean motion resonances were pushed through the surrounding planetesimal disc. They swept up objects in much the same way as a snow plough sweeps up snow. Malhotra has examined this process numerically, and finds that objects can indeed be trapped in resonances as Neptune moves, and that their eccentricities and inclinations are pumped during the process. This scenario is a natural consequence of angular momentum exchange with the planetesimals and there is really no doubt that angular momentum exchange took place.
However, some researchers are unsure whether Neptune moved out as opposed to in, and question the distance this planet might have moved. They also assert that the inclination of Pluto is larger than typical of the objects in Malhotra's simulations.
The dynamical situation is presently unclear, but the "moving planets" hypothesis appears as good as any, and better than most.

Classical Kuiper Belt Objects
A majority of the observed Kuiper Belt Objects maintain large distance from Neptune even when at perihelion. The archetypal "Classical KBO" is 1992 QB1. Such objects are able to survive for the age of the solar system without the special protection offered by resonances to the Plutinos, simply because they are already Neptune-avoiding. The CKBO's are found mostly with semi-major axes between about 42 and 48 AU. They are "classical" in the sense that their orbits have small eccentricities as is expected of bodies formed by quiet agglomeration in the early Solar system.
Inclinations
The inclinations of the Classical KBO's range up to very high values (1996 RQ20 and 1997 RX9 have i > 30 degrees). This suggests that the inclinations have been excited by something yet to be identified. Two ideas have been suggested for the excitation mechanism:
• A few massive planetesimals might have been scattered into the Kuiper Belt in the early days by Neptune. These objects could excite the inclinations of the CKBO's. One problem with this theory is that massive planetesimals (they would have to approach Earth mass in order to be effective) would also disturb and depopulate the resonances. The fact that we see many Plutinos is evidence against the action of massive planetesimals.
• A passing star might have stirred up the CKBO's. Proponents of this idea claim, based on numerical simulations, that the Classical objects can be excited while the Plutinos remain relatively undisturbed. One obvious problem of the external disturbance theory is that passing stars rarely pass close enough to the sun (a miss distance of a few 100 AU is required). However, it is possible that the sun was formed with other stars in a cluster that initially might have been very dense. In this case, the early rate of close stellar passages might have been much higher than at present.

Scattered Kuiper Belt Objects
Some KBO's possess large, eccentric, inclined orbits that have perihelion distances near 35 AU. The archetypal "Scattered Kuiper Belt Object" is 1996 TL66 , discovered as part of a 50 square degree survey using the University of Hawaii 2.2-m telescope on Mauna Kea. In February 1999, 3 more examples of SKBO's (1999 CV118, CY118 and CF119) were discovered in a deeper wide field survey. As surveys progressed the number of SKBO's has risen dramatically, so that now clearly can be seen that the SKBO's are a distinct population in the Kuiper Belt, separate from the CKBO'S and Plutinos. It is expected that more SKBO's will be discovered as improved technology allows scientists to probe larger areas of the ecliptic sky to deeper limiting magnitudes.
Population
The 35 AU perihelion distances allow Neptune to exert weak dynamical control over the SKBO's. On billion year timescales, perihelic disturbances by Neptune will change the orbit parameters from their present values.
The SKBO's form a fat doughnut around the Classical and Resonant KBO's, extending to large distances. 1999 CF119 has an aphelion distance near 200 AU, showing that the SKBO doughnut extends to at least this distance. Eventually, much larger orbits will be found.
There is, however, an important problem with finding SKBO's with very large aphelion distances. Such objects spend only a small fraction of each orbit close enough to the sun to be detected in ground-based observational surveys. 1999 CF119, for example, would be undetectable in the survey in which it was discovered for more than 90% of each orbit. This is why large sky areas must be studied in order to find SKBO's.
In fact, SKBO's account for only 3 to 4% of the known Kuiper Belt Objects but, because of observational problems, this is a strong lower limit to the abundance of these objects.
Origin
How did the SKBO's get their eccentric, looping orbits?
Fernandez (1980) suggested that planetesimals might be scattered into this type of orbit in the early days of the solar system. KBO's that approach Neptune closely are generally scattered away on short (million year) timescales. Many are passed to the dynamic control of other planets, ultimately to be lost from the solar system by ejection or by absorption (collision with a planet or the sun).
Planetesimals ejected into very large orbits either escape from the gravitational influence of the sun (and then enter interstellar space) or may be disturbed by the galactic tidal field and by passing stars into orbits in the Oort Cloud. Objects scattered to the few 100 AU aphelion distances seen in the SKBO's are immune to galactic and stellar tides, and so remain in a tightly bound swarm (the fat doughnut) surrounding the solar system. Numerical simulations of this process by Duncan and Levison (1997) show this process in operation.
Source of Short-Period Comets
The dynamical involvement with Neptune means that the SKBO's are a potential source of short-period comets. Occasional Neptune disturbances can deflect SKBO's to planet-crossing orbits. Some of these bodies may find their way into the inner solar system, where sublimation of embedded ices will lead to their classification as comets. In part because the SKBO population is very uncertain, the ratio of short-period comets delivered from the resonances to those from the scattered disk is highly uncertain.


How are Kuiper Belt Objects formed?
Kuiper Belt Objects are formed in the same way as any other object in our Solar system: by accretion.
It is likely that the Kuiper Belt Objects are extremely primitive debris from the formation of the Solar system.
The inner, dense parts of the proto-planetary disc condensed into the major planets, probably within a few millions to tens of millions of years.
The outer parts were less dense, and accretion progressed slowly. Evidently, a lots of small objects were formed.

As far as we can tell little is known about the exact conditions of the formation of KBO's.
We weren't able to find much information about this subject yet.